Treatment of metal surfaces



United rates atent hfice 3,057,764 Patented Oct. 9, 1962 3,057,764TREATMENT OF METAL SURFACES Mitchell A. La Botla, East Detroit, andCharles R. Wiese,

Detroit, Mich, assignors to General Motors Corporation, Detroit, Mich, acorporation of Delaware No Drawing. Fiied Aug. 18, 1959, Ser. No.834,401 9 Claims. (Q1. 156-6) This invention relates to the shaping ofmetal parts and more particularly to a bath solution and process forshaping metal parts by controlled chemical dissolution.

In recent years it has increasingly become more apparent thatconventional methods of machining parts, particularly aircraft parts,have become inadequate for obtaining present day needs. Various methodsof fabricating aircraft parts have been employed in the past to obtainhigh strength, exceedingly lightweight articles. However, theconventional methods still do not readily permit the manufacture ofparts having the most desirable strength-to-weight ratios. Moreover,many of the metals currently being employed because of high strength andcorrosion resistance at elevated temperatures are exceptionally hardand, therefore, difficult to conventionally machine. Conventionalmachining operations, such as milling, grinding, etc. are accomplishedvery slowly with these relatively hard materials and, accordingly,machining such materials to the degree required to obtain optimumstrength-to-weight ratios is expensive and tedious.

In the manufacture of hollow turbine buckets for a jet engine, forexample, corrosion-resistant, extremely hard alloys of nickel or cobaltare frequently used. These turbine buckets need only have exceedinglythin walls for strength purposes and such thin walls are especiallydesired for efficient air cooling of the blades. Moreover, the outersurface of the turbine bucket should be as smooth as possible to reducefriction with gaseous streams passing over the bucket. The fabricationof such a turbine bucket by conventional techniques is exceedinglydifficult, time consuming and costly.

Parts made from metals, such as stainless steel, nickel and cobalt basealloys referred to above, frequently are formed by casting, molding orthe like wherein an undesirable projection frequently results at theparting line of the forming members. Similarly, during conventionalmachining operations a burr is likely to result on the periphery of themachined surface. These undesirable projections on the surface of thepart must subsequently be removed by finishing operations. However, suchfinish operations are additionally time consuming and costly.

It is a primary object of this invention to provide a bath solution andprocess for treating a metal surface in such a manner as to dissolve themetal of the surface at a rapid rate while concurrently maintaining orimproving the smoothness of the surface.

It is universally recognized that the surface finish of a part which issubjected to physical and thermal stresses should be exceptionallysmooth. It is expected that the extremely smooth surface finishes arenot only desirable when the part is to be used in a gas stream but, inmost instances, necessary to obtain optimum resistance to fatigue andcorrosion. Our invention provides a means whereby the surface of a partmade of metals, such as stainless steels, nickel base alloys andparticularly cobalt base alloys, is subjected to a controlled etchingwhich can be used to improve the smoothness or surface finish of thepart to remove undesirable surface projections and to produceexceedingly thin structures.

Accordingly, it is a further object of our invention to provide a meanswhereby such materials can be produced economically and rapidly undercommercial production conditions to produce optimum strength-to-weightratios heretofore frequently unobtainable and to concurrently obtainsurface finishes which were only laboriously obtained by conventionalfinishing techniques.

We have now found that a cobalt base alloy can be satisfactorilymachined by chemical dissolution in an aqueous bath containing ferricchloride, hydrochloric acid, hydrofluoric acid, nitric acid and iodine.By surface finis as used herein, we refer to the degree of smoothness orroughness of a given surface as distinguished from the degree ofreflectivity of a polished surface.

By the term controlled etching, as used herein, we refer to the etchingof a metal surface in such a manner as to uniformly dissolve the metalwithout selectively etching grain boundaries or grain of the metal, thusat least maintaining the original surface condition. Such controlledetching will concurrently attack projections or peaks on the surface toa greater degree than recesses or valleys, thereby concurrentlysmoothing out or leveling the surface.

For the purposes of our invention by the term stainless steel we intendto encompass various ferrous base alloys containing approximately 10% ormore chromium, such as alloys analogous to the SAE 3-00 and SAE 400series stainless steels. References made herein to nickel base alloysand cobalt base alloys are intended to include those metals,respectively, containing more than 50%, by weight, nickel and more than50%, by weight, cobalt.

Many stainless steels and nickel base alloys may be chemically machinedwith the bath composition of our invention. However, far greateradvantages are realized in the treatment of cobalt base alloys.

The following Table I contains examples of suitable high temperaturecobalt base alloys which may be satisfactorily chemically machined inaccordance with the present invention, the compositions being listed inpercent, by weight:

Table 1 Example 1 Example 2 Example 3 Carbon. 0.20-0.35 0. 20 0 32-0. 42Manganese 1 00-200 0 (SO-1.80 Silicon 1 0. 60 0 30-0. 90 Sulfur 1 0.40Chromium 25. 00-30. 00 20.00-22.00 19 00-21. 00

ickel 1.50-3.50 18.00-22.00 19 00-21. 00 Molybdenunu 2. 50-3. 25 3.50-4. 50 Tungsten. 2.00-3.00 3. 50-5. 00 Oolumbiu 0. -1. 25 8 00-4. 50Nitrogen. 0. 08-0. 16 Iron 1 5.00 Cobalt 18. 00-22. 00 40. 00-44. 00

1 Max.

2 Balance.

We have also found that the alloys listed in the following Table II, thecompositions of which are listed in percent, by weight, are so resistantto uniform chemical attack that they cannot be satisfactorily chemicallyetched without the use of our invention:

Table II Example 1 Example 2 0. 45-0. 60 0. 12 23 Oil-28.00 20.00 600-9. 00 14.00 9.12 10.00 1 2.00 1 2.00

' Max. 2 Balance.

As previously indicated the bath of our invention encompasses an aqueoussolution containing ferric chloride,

hydrochloric acid, nitric acid, hydrofluoric acid and iodine. The ferricchloride used in the bath can be added as a solid or in aqueoussolution. We prefer to employ a commercially available, comparativelylower cost aqueous ferric chloride solution having a density of a 42Baum (about 546 grams per liter). When chemically machining an alloy,such as described in Table II, We prefer to employ about 400 millilitersof a 42 Baum ferric chloride solution (about 219 grams of ferricchloride) in every 900 milliliters of bath solution. Thus, for thesealloys we prefer to employ a bath solution containing the equivalent ofabout 44%, by volume, of a 42 Baum ferric chloride solution. In general,however, satisfactory results can be obtained if the ferric chloride ismaintained in amounts produced by additions of about 360 milliliters to440 milliliters in every 900 milliliters of bath solution. In certaininstances, it may be preferred to use as low as about 35% or as high asabout 55%, by volume, of the 42 Baum solution in the bath solution.

Similarly, the amount of hydrochloric acid in our solution is somewhatvariable. However, for alloys, such as listed in Table II, we prefer toemploy about 300 milliliters, about 33%, by volume, of concentratedhydrochloric acid (specific gravity 1.19) in forming every 900milliliters of bath solution. Generally, satisfactory results can beobtained with our bath solution when the bath solution contains about26% to 40% of concentrated hydrochloric acid.

The nitric acid content in about 900 milliliters of bath solution ispreferably equivalent to approximately 100 milliliters of concentratednitric acid (specific gravity 1.42). In certain instances, we may preferto employ a bath solution containing as low as about 8% or as high asabout 14%, by volume, of concentrated nitric acid.

Of course, the amount of water used in forming the bath solution willvary according to the proportions of other materials which are used informing the bath. Since it is preferred to add concentrated acid toWater rather than vice versa, we prefer to add suflicient amounts ofother bath ingredients to a given volume of water to obtain the desiredbath proportions.

Some cobalt base alloys have a tendency to be extremely non-reactive tohighly corrosive acids, such as those used in our bath solution. Analloy of the type described in Example 1 of Table II is virtuallynon-reactive to the bath solution unless iodine crystals are added tothe bath. The amount of iodine which is employed is variable, but therate of the reaction with this alloy as Well as many others primarilydepends upon the amount of reactive iodine which is present in thesolution. Even small amounts of iodine present in the solution areeffective in making such a metal reactive to the solution althoughhigher amounts of about grams are preferred to accelerate the reactionof the solution with the alloy. Amounts of crystalline iodine in excessof 5 grams do not appear to dissolve in the solution and, accordingly,do not appear to provide any material advantages. In general, formaximum reactivity, We prefer to employ about 5 grams of crystallineiodine in every 900 milliliters of bath solution.

It has been found that occasionally an especially nonreactive metal,such as the cobalt alloys listed in Table III, will passivate after aperiod of reaction with our solution. In such instance, it is necessaryto remove the part from the bath solution, rinse in cold water, vaporblast the surface of the metal and then dip the part in hydrochloricacid, preferably concentrated, for a few seconds. The part can then bereimmersed in the bath solution whereupon further reaction with thesolution will occur. The specific character of the vapor blast is onlyas critical to preserving the finish as in any other treatment involvinga vapor blast and can be accomplished in the known and accepted manner.

In the vapor blast treatment the length of time that thevapor-liquid-abrasive stream should be applied depends upon the hardnessof the particular metal being treated. Periods in excess of about oneminute to two minutes usually should be avoided. An abrasive liquidslurry containing one volume of an abrasive of about 325 mesh to 3volumes of liquid can be aspirated into a vapor stream having a pressureof at least about 40 pounds per square inch. Any suitable liquid can beused, such as water or a volatile organic solvent.

If excessive amounts of metal removal are desired, passivation of themetal part may occur during the prolonged duration of removal.Accordingly, activation in the above-described manner may be repeatedlynecessary in order to complete metal removal to the desired depth.Alloys, such as the examples in Table I or Table II need not beactivated in most instances.

It is generally preferred to clean the metal surface prior to subjectingit to electrical dissolution treatments. Satisfactory results areobtainable when the part is cleaned by degreasing it in atrichloroethylene vapor at a temperature of approximately 180 F. in thenormal and accepted manner. In some instances one of the manycommercially available di-phase cleaners, which is a stable emulsion ofan organic cleaner and an alkali cleaner, might be used.

After degreasing, the part is dried and then, suitably supported,immersed in the chemical machining bath. The part is preferablypositioned in the machining solution in such a manner as to avoidnon-uniform dissolution of the surfaces. Gas which may be generatedduring the dissolution of the metal part can accumulate in recesses ofhorizontal areas of the part so as to interfere with uniform chemicaldissolution of the entire surface. When chemically machining an articlehaving a planar configuration, such as a panel, it is desirable tosupport the panel in the machining solution in a vertical attitude.

On the other hand, articles of a more complicated configurationcontaining complex contours and recesses may not be suitably maintainedin any position which will entirely inhibit collection of the generatedgases and formation of gas pockets. For these and other types ofarticles it may be desirable to chemically machine the parts in aplurality of steps in which portions of the part are masked from thesolution. For example, the top of such an article can be chemicallymachined while its lower surface is masked with a suitable stop-offmaterial. When sufficient metal removal of the upper surface isobtained, the part is removed from the solution, rinsed and the stop-offremoved to expose the protected surface. The machined surface is thenmasked and the part reimmersed for completion of the chemical machiningof the part. Of course, on reimmersion the part is inverted as comparedto the previous machining operation so that the masked surface is on thebottom of the part.

The temperature at which the bath is operated is not especially criticaland, in some instances, temperatures as low as room temperature can beused. Relatively high bath temperatures accelerate the reaction of themetal with the solution and, accordingly, are desired. Accordingly, Wehave found that bath temperatures above about F. are most desirable withthe upper temperature limit, of course, being somewhat below the boilingpoint of the solution to avoid excessive vaporization.

The duration of the machining operation is primarily dependent upon thedesired depth to which one desires to etch. In determining the preferredduration of etching one must give consideration, of course, to the factthat projections on a part tend to dissolve at a greater rate thanrecessed areas contributing to a dimensional loss. Dissolution durationslimited by such dimensional loss can be extended by initially formingthe part to compensate for faster metal removal in areas of aprojection. Analogously, dimensional loss can be inhibited by usingmaskants and the like, such as are well known in the etching art.

Although this invention has been described in connection with certainspecific examples thereof, no limitation is intended thereby except asdefined in the appended claims.

We claim:

1. An aqueous bath for the chemical controlled etching of a metal fromthe group consisting of stainless steel, nickel base alloys and cobaltbase alloys, said bath consisting essentially of the equivalent of about35% to 55% of a 42 Baum aqueous fenric chloride solution, about 26% to40% of concentrated hydrochloric acid and about 8% to 14% ofconcentrated nitric acid, all proportions by volume.

2. An aqueous bath for the chemical controlled etching of the surface ofa metal from the group consisting of stainless steel, nickel base alloysand cobalt base alloys, said bath consisting essentially ofapproximately 35% to 55% of a 42 Baum aqueous ferric chloride solution,about 26% to 40% of concentrated hydrochloric acid, about 8% to 14% ofconcentrated nitric acid, all proportions by volume, and sufficientiodine to at least substantially maintain surface smoothness.

3. An aqueous bath for the chemical controlled etching of the surface ofa metal comprising, by weight, about 20% to 30% chromium, 2% to 14%tungsten, 1% to 21% nickel, 0.6% maximum carbon, 5% maximum iron and thebalance cobalt, said bath consisting essentially of approximately 35% to55% of a 42 Baum aqueous ferric chloride solution, about 26% to 40% ofconcentrated hydrochloric acid, about 8% to 14% of concentrated nitricacid, all proportions by volume, and sufficient iodine to at leastsubstantially maintain surface smoothness.

4. The method of chemically machining a part made of a metal from thegroup consisting of stainless steel, nickel base alloys and cobalt basealloys, said method comprising applying to said part an aqueous bathsolution consisting essentially of approximately 35 to 55 of a 42 Baumaqueous ferric chloride solution, about 26% to 40% of concentratedhydrochloric acid, about 8% to 14% of concentrated nitric acid, allproportions by volume, and sufficient iodine to at least substantiallymaintain surface smoothness.

5. The method of chemically machining a cobalt base alloy part Whichcomprises applying to said part an aqueous bath solution consistingessentially of approximately 35% to 55% of a 42 Baum aqueous ferricchloride solution, about 26% to 40% of concentrated hydrochloric acid,about 8% to 14% of concentrated nitric acid, all proportions by volume,and sufficient iodine to at least substantially maintain surfacesmoothness and etching said surface to a depth of about 0.003 inch perminute while simultaneously at least substantially maintaining surfacesmoothness.

6. The method of chemically machining a cobalt base alloy part whichcomprises applying to said part an aqueous bath solution consistingessentially of approximately 35% to 55% of a 42 Baum aqueous ferricchloride solution, about 26% to 40% of concentrated hydrochloric acid,about 8% to 14% of concentrated nitric acid, all proportions by volume,and sufiicient iodine to at least substantially maintain surfacesmoothness, subsequently vapor blasting said surface, applying to saidsurface a concentrated hydrochloric acid solution and subsequentlyreapplying said bath solution to said surface.

7. The method of chemically machining a part made of a metal comprising,by weight, about 20% to 30% chromium, 2% to 14% tungsten, 1% to 21%nickel, 0.6% maximum carbon, 5% maximum iron and the balance cobalt,said method comprising applying to said part an aqueous bath solutionconsisting essentially of approximately 35% to 55 of a 42 Baum aqueousferric chloride solution, about 26% to 40% of concentrated hydrochloricacid, about 8% to 14% of concentrated nitric acid, all proportions byvolume, and sufficient iodine to at least substantially maintain surfacesmoothness.

8. The method of chemically machining a part made of a metal comprising,by weight, about 23% to 28% chromium, 6% to 9% tungsten, 9% nickel,0.45% to 0.60% carbon, 2% maximum iron and the balance cobalt, saidmethod comprising applying to said part an aqueous bath solutionconsisting essentially of approximately 35 to 55% of a 42 Baum aqueousferric chloride solution, about 26% to 40% of concentrated hydrochloricacid, about 8% to 14% of concentrated nitric acid, all proportions byvolume, and sufficient iodine to at least substantialy maintain surfacesmoothness, subsequently vapor blasting said surface, applying to saidsurface a concentrated hydrochloric acid solution and subsequentlyreapplying said bath solution to said surface.

9. The method of chemically machining a cobalt base alloy part whichcomprises applying to a surface of said part an aqueous bath solutionconsisting essentially of approximately 35% to 55% of a 42 Baum aqueousferric chloride solution, about 26% to 40% of concentrated hydrochloricacid, about 8% to 14% of concentrated nitric acid, all proportions byvolume, and sufficient iodine to at least substantially maintain surfacesmoothness, etching said surface to a depth of about 0.003 inch perminute while simultaneously at least substantially maintaining surfacesmoothness, thereafter vapor blasting said surface, applying to saidsurface a concentrated hydrochloric acid solution and subsequentlyreapplying said bath solution to said surface.

References Cited in the file of this patent UNITED STATES PATENTS1,776,535 Bekk Sept. 23, 1930 2,266,430 Matthews Dec. 16, 1941 2,684,892Savlnier July 27, 1954 2,746,848 Jones May 22, 1956 2,890,944 Hays June16, 1959 FOREIGN PATENTS 513,130 Canada May 24, 1955 OTHER REFERENCESMoneypenny: Stainless Iron & Steel, page 512, pub. Chapman & Hall,London, cpw. 1931, 2nd ed. rev.

Metals Handbook, 1948 ed., page 395, Table II, etch bath 8 (A) and (B).

4. THE METHOD OF CHEMICALLY MACHINING A PART MADE OF A METAL FROM THEGROUP CONSISTING OF STAINLESS STEEL, NICKEL BASE ALLOYS AND COBALT BASEALLYS, SAID METHOD COMPRISING APPLYING TO SAID PART AN AQUEOUS BATHSOLUTION CONSISTING ESSENTIALLY OF APPROXIMATELY 35% TO 55% OF A 42*BAUME'' AQUEOUS FERRIC CHLORIDE SOLUTION, ABOUT 26% TO 40% OFCONCENTRATED HYDROCHLORIC ACID, ABOUT 8% TO 14% OF CONCENTRATED NITRICACID, ALL PROPORTIONS BY VOLUME, AND SUFFICIENT IODINE TO AT LEASTSUBSTANTIALLY MAINTAIN SURFACE SMOOTHNESS.